A snapshot function of a storage system is a function to retain data of a volume of a certain point of time in the past. The volume being the source of the snapshot is called a PVOL (Primary Volume), and the volume retaining past data is called a SVOL (Secondary Volume). For efficiency of capacity, each SVOL has only the actual data of PVOL or an SVOL acquired newly than itself and the difference data therefrom stored in the pool. The storage system refers to a difference management information and a VOL allocation management information, and specifies the storage position of SVOL data.
When a snapshot function is adopted in a migration source volume of a migration source storage system, it is desirable that the processing of the snapshot function is continued in the migration destination storage system.
According to an example for migrating data being subjected to snapshot management, a PVOL, an SVOL and a pool are prepared in the migration destination storage system, and SVOL data of the oldest generation (first generation) is copied from the migration source storage to the PVOL of the migration destination storage. Next, the difference management information between the first generation SVOL and an SVOL of the second oldest generation (second generation) and the VOL allocation management information are referred to in the migration source storage system, and only the data having difference out of all the second generation SVOL data are transferred to the PVOL of the migration destination storage. The transferred data is written into the PVOL of the migration destination storage, and the first generation SVOL is restored by the snapshot function of the migration destination storage system. A technique of migrating a snapshot function is provided (refer for example to patent literature 1) in which the above-described process is repeatedly performed to restore the snapshot of the migration source storage system.